The former The Wonder Years star, 39, suffered the injury during rehearsals on Friday (25Apr14), but she refused to let it hinder her progress on the Tv competition.
Prior to Monday's live broadcast, MCKellar told her Twitter.com followers she was "a little nervous" about dancing with a broken rib, but vowed, "I'm gonna try my best".
She managed to impress the judges, including guest Ricky Martin, with her salsa - and was saved from elimination. Instead, it was Glee actress and reality Tv star NeNe Leakes who became the latest celebrity to leave the show.
Meanwhile, MCKellar wasn't the only competitor suffering on Monday's show - U.S. paralympic snowboarder Amy Purdy was hospitalised after her performance due to a back injury.